Existing model of distribution system security region is based on interconnections of substation transformers. This model simplifies the detailed topological connections of feeders, which results in that the security boundary cannot match results of N?1 point-by-point simulation. To solve this problem, this paper firstly proposes a new model of distribution system security region based on interconnections of feeders, which considers both transformer N?1 and feeder N?1 contingency and its result includes load of feeder sections. The model can also be applied to the complex distribution networks with multi-links. Secondly, a parsing algorithm of security region boundary is presented based on the model, and its results are hyperplan expressions formed by set of inequalities. Thirdly, to validate the accuracy of security region model, two kinds of N?1 simulation proof methods are designed. Results of a study case show that the proposed model can describe distribution security boundary more precisely and rapidly, which can lay a theoretical foundation for precise guidance of secure and efficient operation of distribution system.